Method of treating magnesium articles



UNITED STATES PATENT OFFICE METHOD OF TREATING MAGNESIUM ARTICLES John A. Gann and William H. Gross, Midland,

Mich, assignors to The Dow Chemical Company, Midland, Mich., a corporation of Michi- No Drawing. Application February 18, 1935, Serial No. L014 2 Claims.

The invention relates to a method of chemically treating the surface of magnesium and magnesium alloy articles to remove surface impurities and prepare the surface for painting, etc. The

5 invention is especially well adapted to the removal of surface contaminations resulting from autogenous welding operations, particularly when a flux is employed in the welding. This application is a division of our copending application,

issuing as U. S. Patent 1,992,205 on February 26,

It has been proposed heretofore to subject articles of magnesium and its alloys to the action of a 10 to 30 per cent aqueous solution of sulphuric l5 acid to remove superficial impurities. It has been proposed also to employ an aqueous solution of hydrochloric acid for the same purpose. However, we have found that, although such acid solutions may remove superficial impurities, the

amount of metal dissolved by the acid at the same time is so excessive and the resulting alteration of the dimensions of the article so great that the use of the foregoing acid solutions is not practically useful. One of the objects of the invention is to provide a method of cleaning articles of magnesium and its alloys by subjecting them to the action of an acid bath whereby superficial impurities are removed while excessive attack upon the metal by the acid is prevented. Another object is to provide a method of preparing the surface of the metal to receive paint, varnish, and the like. Other objects and advantages will appear as the description proceeds.

We have discovered that aqueous solutions containing lmdrochloijg cid are very efiective for removing superficial impurities normally present on articles of magnesium and its alloys, without excessive attack upon the metal, if to the solution we add a substantial proportion of a soluble chromate, such as hromic acid, or sa lts thereof, including chromates a pj gi chr omatemoi pgaa saama elsiuin amn nnmmaaqalkali .mslals. and a substantial amount of a soluble mgg nesium :Et, such as magnesium r; g, magnesiunrpi;

trate, or magnesium sulphate. In addition to fin Ti'ifig superficial impurities such solutions so alter the character of the metal surfaces as to cause paint, varnish, or the like subsequently applied thereto to adhere more tenaciously than when the surface of the metal is treated with a solution of the acid alone. The invention, then, consists in the method of treating articles of magnesium and its alloys hereinafter fully described and particularly pointed out in the claims,

the following description setting forth in detail several modes of carrying out the invention, such disclosed modes illustrating but several of the various ways in which the principle of the invention may be used.

In carrying our invention into effect the mag- 6 nesium articles to be treated are subjected to the action of an aqueous solution. the composition of which is described below, by spraying the solution onto the metal, or by immersing the metal in the solution, or in any other manner in which the 10 solution and the article may be brought into contact for a suflicient length of time to dissolve the superficial impurities. Thereafter the article is Washed with water and then dried. The treating bath contemplated by our invention is prepared 5 by forming an aqueous solution containing from 2 to 20 parts of hydrogen chloride (HCl), from 5 to 40 parts of a soluble chromate such as chromic acid or salts thereof, including the chromates and bichromates of magnesium, calcium, ammonium 20 and alkali metals, and from 5 to 40 parts of a soluble magnesium salt such as magnesium sulphate, magnesium nitrate, and magnesium chloride, per 100 parts of Water. The following examples are illustrative of bath compositions made 25 up according to our invention and their action on a magnesium alloy (4 per cent A1, 0.3 per cent Mn, balance Mg) dipped into, immersed in, or otherwise brought into contact with the solution.

Example 1 In a bath composed of 29 cc. of concentrated hydrochloric acid (36 per cent HCI, balance water), 10 grams of magnesium chloride (MgCl2.6H2O), and enough water to make 100 cc. 35 of solution we immersed strips of magnesium alloy rolled sheet for 1 minute. The rolled magnesium alloy strips employed were coated with the usual oxide film resulting from heating during rolling operations. After immersion the strips 40 were removed, washed and dried. On examining the so treated strips we found that the surface impurities were completely removed leaving a bright, smooth surface and the thickness of metal dissolved was 0.029 inch. For comparison we de- 45 termined the amount of metal dissolved when similar magnesium alloy strips were imersed in a hydrochloric acid bath of the same concentration as in the abovementioned bath, but without adding magnesium chloride thereto. In this case 50 we found that the metal thickness was reduced as much as 0.044 inch. Thus, the presence of a magnesium chloride in the hydrochloric acid bath reduced the attack on the metal about 34 per cent.

- By including a soluble chromate in the hydro- -""chloric acid bath the attack on the metal by the acid may be still further reduced as illustrated in the following example.

Example 2 In a bath composed of 3 parts of hydrogen chloride, 10 parts of sodium bichromate (Na2Cr20m2H2O), and 100 parts of water we immersed for 30 seconds similar superficially contaminated magnesium alloy rolled strips, then removed, washed, and dried them. On examination the so treated strips were found to be clean and the amount of metal dissolved only 0.4 per cent by weight. When similar strips were immersed for 30 seconds in the hydrochloric acid solution of the same concentration as in the abovementioned bath, but not containing sodium bichromate, the amount of metal dissolved was more than 4 per cent by weight.

The following example illustrates the effect of employing a hydrochloric acid bath containing both a soluble magnesium salt and a soluble chromate.

Example 3 In a bath composed of 6 parts of hydrogen chloride, 10 parts of magnesium sulphate (MgSOUIHzO). 10 parts of sodium bichromate (NazCrz0a.2HzO), we immersed for 30 seconds similar rolled strips of magnesium alloy. After such treatment, washing and drying, the strips were found to be free from superficial impurities.

The loss in weight of metal due to the dissolving action of the acid was only 2.5 per cent. In a similar test employing a bath containing hydrogen chloride and sodium bichromate at the same concentration as in the abovementioned bath, but containing no magnesium sulphate, the loss in weight of metal was 2.9 per cent. In another similar test employing a bath containing hydrogen chloride and magnesium sulphate at the same concentration, but no sodium bichromate,

. the loss in weight was 7.5 per cent. When we similarly employed a bath containing hydrogen chloride only at the same concentration, the loss in weight was 14.3per cent. Thus, while either magnesium sulphate or sodium bichromate reduces the attack of hydrochloric acid on the metal, the presence of both of these salts in the bath reduces the attack by the acid to a greater extent than either salt alone.

The length of time to subject the magnesium article to the action of the solution may be varied according to the amount of superficial impurities to be removed, the concentration of the solution, and the temperature of the solution. The higher the temperature or concentration of the solution the more rapid is its action. When working at room temperature usually about 1 minute is a suflicient length of time of contact when the proportion of hydrogen chloride is 2 to 6 parts, and that of soluble chromate 5 to 10 parts, per 100 parts of water. With higher concentrations about 30 seconds is a sufficient time of contact. 5 When a soluble magnesium salt in amount between 5 and parts per 100 parts of water is added to the foregoing bath, the cleaning action of the solution is improved and the amount of metal dissolved by the acid is substantially reduced. A bath composition which is suitable for cleaning, especially dirty magnesium and magnesium alloy articles, contains from 6 to 17 parts of hydrogen chloride, 20 to parts of a soluble chromate with or without the addition of a soluble magnesium salt, although the addition of the latter in amount from 5 to 30 parts still further improves the cleaning action of the solution.

After a magnesium or magnesium alloy article has been subjected to the action of our solutions as described, washed and dried, coatings of paint or varnish and the like, when applied to the metal, adhere very much more tenaciously and for a longer time than when the surface is previously treated with an aqueous solution of either hydrochloric acid or sulphuric acid alone.

Although in the foregoing examples we described cleaning baths containing sodium bichromate, as an example of a soluble chromate,

and magnesium chloride and sulphate, as examples of soluble magnesium salts, we do not wish to be limited to these specific salts as other equivalent soluble chromates and soluble magnesium salts may be employed as aforesaid.

By the term magnesium used herein and in the appended claims we mean not only magnesiurn, but also alloys thereof wherein magnesium is the predominating constituent.

Other modes of applying the principle of our invention may be employed instead of those ex- 9 plained, change being made as regards the method herein disclosed, provided the step stated by any of the following claims or the equivalent of such stated step be employed.

We therefore particularly point out and distinctly claim as our invention 1. In a method of treating a magnesium article, the step which consists in subjecting the ar-. ticle to the action of an aqueous solution containing hydrogen chloride 2 to 20 parts, a soluble chromate 5 to 40 parts, and a soluble magnesium salt 5 to 40 parts, per parts of water.

2. In a method of treating a magnesium article, the step which consists in subjecting the ar- WILLIAM H. GROSS. 

